Three-dimensional gallium nitride (GaN) micro- and nanostructures have the potential to boost the performance of GaN-based electronic and optoelectronic devices (Chung, K. et al. (2010) “Transferrable GaN Layers Grown on ZnO-Coated Graphene Layers for Optoelectronic Devices,” Science 330:655; Matsubara, H. et al. (2008) “GaN Photonic-Crystal Surface-Emitting Laser at Blue-Violet Wavelengths,” Science 319:445-447; Fasol, G. (1996) “Room-Temperature Blue Gallium Nitride Laser Diode,” Science 272:1751; Morkoc, H. & Mohammad, S. N. (1995) Science 267:51). These structures have significant benefits including large active surface area, enhanced light extraction efficiency, greater carrier confinement, lower defect density, and reduction of detrimental spontaneous polarization effects as compared to conventional planar thin-film devices. The latter benefit arises as p-n junctions can be realized on non-polar and semi-polar surfaces.
GaN-based micro- and nanopillar (NP) core-shell structures have been produced by a number of different techniques including: (1) catalyst-assisted metal-organic vapor phase epitaxy (MOVPE) (Qian, F. et al. (2005) “Core/Multishell Nanowire Heterostructures as Multicolor, High-Efficiency Light-Emitting Diodes,” Nano Lett. 5:2287-2291), (2) selective area epitaxy (SAE) of nanorods using MOVPE through a dielectric mask and subsequent conformal shell growth (Hong, Y. J. et al. (2011) “Visible-Color-Tunable Light-Emitting Diodes,” Adv. Mater. 23:3284; Waag, A. et al. (2011) “The Nanorod Approach: GaN NanoLEDs for Solid State Lighting,” Phys. Status Solidi C 8:2296), and (3) a combination of molecular beam epitaxy (MBE) grown n-type GaN nanowires and Hydride Vapor Phase Epitaxy (HVPE) grown p-GaN conformal shells (Sanders, A. et al. (2011) “Homoepitaxial n-core: p-shell Gallium Nitride Nanowires: HVPE Overgrowth on MBE Nanowires,” Nanotechnology 22:465703). Despite superior material quality in structures fabricated from such methods, lack of large-area control of dimensions, morphology, and orientation remain problematic for device applications.
In producing NP arrays for device applications, the structures and/or growth-techniques should possess the following characteristics: high-aspect ratio, large-area uniformity in dimensions and orientation, scalable fabrication and growth methods, and precise control of shell morphology, microstructure, and dopant concentration. Development of NP arrays with high aspect ratios (10 and higher) and diameters in the range of 150 nm to 250 nm require particularly careful design of the etch process and selection of the mask material. It is also important to analyze and control the surface defects as well as strain in the fabricated NP arrays. Obtaining smooth and defect-free sidewalls is also important for the subsequent selective epitaxial overgrowth.
Traditionally, research on nitride nanostructures focused on bottom-up fabrication methods (Qian, F. et al. (2008) “Multi-quantum-well Nanowire Heterostructures for Wavelength-controlled Lasers,” Nature Mater. 7:701-0706; Yan, R. et al. (2009) “Nanowire Photonics,” Nature Photon. 3:569; Johnson, J. C. et al. (2002) “Single Gallium Nitride Nanowire Lasers,” Nature Mater. 1:106-110; Qian, F. et al. (2005), supra, Nano Lett. 5:2287; Aluri, G. S. et al. (2011) “Highly Selective GaN-nanowire/TiO2-nanocluster Hybrid Sensors for Detection of Benzene and Related Environment Pollutants,” Nanotechnology 22:295503). Top-down fabrication methods provided an alternative route for producing micro- and nanopillar (NP) arrays from epitaxially grown thin films (Naureen, S. et al. (2011) “High Optical Quality InP-based Nanopillars Fabricated by a Top-down Approach,” Nano Lett. 11:4805-4811; Naureen, S. et al. (2013) “Generation of Substrate free III-V Nanodisks from User-defined Multilayer Nanopillar Arrays for Integration on Si,” Nanotechnology 24:225301; Naureen, S. et al. (2012) “Top-Down Fabrication of High Quality III-V Nanostructures by Monolayer Controlled Sculpting and Simultaneous Passivation,” Adv. Funct. Mater. 23:1620-1627; Sanatinia, R. et al. (2012) “GaAs Nanopillar Arrays with Suppressed Broadband Reflectance and High Optical Quality for Photovoltaic Applications,” Opt. Mater. Express 2:1671-1679; Sanatinia, R. et al. (2012) “Surface Second-Harmonic Generation from Vertical GaP Nanopillars,” Nano Lett. 12:820).
In addition, selective area epitaxy (SAE) techniques used to fabricate nanostructures have significantly advanced in the past few years (Li, S. & Waag, A. (2012) “GaN Based Nanorods for Solid State Lighting,” J. Appl. Phys. 111:071101; Hersee, S. D. et al. (2006) “The Controlled Growth of GaN Nanowires,” Nano Lett. 6:1808-1811). GaN columns or rods grown along the c-axis bounded by six {1100} m-plane facets are especially attractive because InGaN quantum wells formed on these facets benefit from the absence of polarization fields (Waltereit, P. et al. (2000) “Nitride Semiconductors Free of Electrostatic Fields for Efficient White Light-emitting Diodes,” Nature 406:865). Arrays of GaN micro- and nanocolumns have been successfully grown by SAE using metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy techniques (Bergbauer, W. et al. (2010) “Continuous-flux MOVPE growth of position-controlled N-face GaN Nanorods and Embedded InGaN Quantum Wells,” Nanotechnology 21:305201; Bertness, K. A. et al. (2010) “Controlled Nucleation of GaN Nanowires Grown with Molecular Beam Epitaxy,” Adv. Funct. Mater. 20:2911). However, although traditional SAE techniques have achieved the desired dimensional and orientation control, they require highly specific growth conditions leading to non-optimal material quality (Wang, G. T. et al. (2011) “Nanowires: Lighting the future” SPIE Newsroom, Jul. 26, 2011, doi:10.1117/2.1201106.003796).
GaN core/shell hexagonal columns have also been realized by inductively coupled plasma (ICP) dry etching of GaN films followed by MOVPE overgrowth to recover the m-plane facets (Fichtenbaum, N. A. et al. (2007) “Metalorganic Chemical Vapor Deposition Regrowth of InGaN and GaN on N-polar Pillar and Stripe Nanostructures,” Jpn. J. Appl. Phys., Part 2 46:L230; Le Boulbar, E. D. et al. (2013) “Facet Recovery and Light Emission from GaN/InGaN/GaN core-shell Structures Grown by Metal Organic Vapour Phase Epitaxy on Etched GaN Nanorod arrays,” Appl. Phys. 114:094302). The combination of nanopatterning and plasma etching to form GaN pillars and subsequent coaxial shell growth has also been attempted to alleviate the problems associated with SAE (Wang, G. T. et al. (2011), supra, SPIE Newsroom, Jul. 26, 2011; Fichtenbaum, N. A. et al. (2007), supra, Jpn. J. Appl. Phys., Part 2 46:L230).
However, significant challenges and problems associated with such conventional fabrication techniques remain, including control of strain, defect reduction, and mitigation of unwanted polarization fields. Accordingly, there is a need for improved fabrication methods and resulting micro- and nanostructures that overcome some or all of the above-noted limitations and/or disadvantages.